1. Field of the Invention
The present invention relates to a method and apparatus for reducing a jitter of a program clock reference of an MPEG signal transmitted according to MPEG (Moving Picture Experts Group) over ATM (Asynchronous Transfer Mode) system. The present invention also relates to an MPEG decoder having the apparatus for reducing the jitter of a program clock reference of an MPEG signal transmitted according to MPEG over ATM system.
2. Description of the Related Art
In the MPEG 2 system standard which is an storage/transmission standard of moving pictures, STC (System Time Clock) is used in an encoder for encoding audio and video signals into code and a decoder for decoding code into audio and video signals. The decoder starts its decoding operation using code in a main buffer and outputs decoded data to a reorder buffer at the time when STC becomes the same as DTS (Decoding Time Stamp) which corresponds to the codes in the main buffer. Further, the decoder outputs the decoded data in the reorder buffer at the time when STC becomes the same as PTS (Presentation Time Stamp) which corresponds to the decoded data. SCR (System Clock Reference) and PCR (Program Clock Reference) are defined as a time stamp for synchronizing STC of the decoder with STC of the encoder. The transmission interval of PCR must be set to 100 msec or less according to MEPG2 standard.
Further, in MPEG2 system, a transport stream (TS) is known as one of transmission streams. The transport stream consist of packets having a fixed length of 188 bytes, and such construction is adapted on the assumption that transmission is performed over ATM system or through digital broadcast system. A payload or adaptation field is allocated to a portion other than a packet header of the transport stream packet. The adaptation field has a function for transmitting information such as PCR and the like and a stuffing function.
A system for transmitting a signal of MPEG2 over ATM is known as MPEG over ATM. When a transport stream is transmitted over ATM, the transport stream is sent to AAL (ATM Adaptation Layer) to be processed into an ATM cell. An AAL1 cell, an AAL2 cell, and an AAL5 cell are selected as the ATM cell. For the processing a transport stream into an ATM cell, there may be considered a case where one transport stream packet is processed into five AAL5 cells, a case where two transport stream packets are processed into eight AAL5 cells and a case where three transport stream packets are processed into twelve AAL5 cells. In connection with these cases, it is an mandatory specification for both the transmission side and the reception side to support the format for processing two transport stream packets into eight AAL5 cells.
An MPEG2 data transmission apparatus disclosed in JPA-9-116563 can perform stream processing so as to greatly reduce the transmission processing time for transmitting MPEG2 data to an ATM channel according to the transport packet system and so as to reproduce the data accurately with respect to the time.
Further, a data transmission method and a data transmission system disclosed in JPA-9-64874 enables high-quality data transmission by excluding the influence of jitter between two transmission apparatuses which are connected to each other through an ATM communication channel.
Still further, an ATM clock reproducing apparatus disclosed in JPA-7-66814 is used in a broad band ISDN (B-ISDN) network when video signals having clocks which are independent of the network are transmitted, and it reproduces clocks in which jitter due to accumulation of residuals caused when the difference between time stamps is rounded to an integer at the reception side is suppressed.
Still further, an apparatus and a method for segmentation and time-synchronization of transmission of multimedia data disclosed in JPA-8-321836 aim to improve the segmentation and the time-synchronization of the multimedia digital data stream.
In the MPEG over ATM for transmitting an MPEG transport stream over ATM, as shown in of FIGS. 2A and 2B, two transport stream packets are processed into eight ATM cells of AAL5. On the other hand, at the reception side, two transport packets are reassembled at the time when eight ATM cells are received, and thus these two transport packets are coupled into a burst data as shown in FIG. 2C. Therefore, in accordance with a condition as to whether a packet in which PCR is inserted is a first packet or a second packet of the burst data, the position of PCR is displaced from its original position, so that jitter occurs in STC generated on the basis of the PCR.
An object of the present invention is to provide an MPEG decoder and a jitter reducing method and apparatus which can generate a system time clock of which jitter is reduced even when the position of a program clock reference in a received MPEG over ATM signal is varied.
According to a first aspect of the present invention, there is provided an apparatus for reducing a jitter of a program clock reference in a transport stream of an Moving Picture Experts Group (MPEG) signal transmitted over Asynchronous Transfer Mode (ATM) system, which comprises: means of correcting PCR to PCRxe2x88x92(Txe2x88x92xcex4) if |PCRxe2x88x92STC| greater than Txe2x88x92xcex4xe2x88x92xcex94 is true and sign(PCRxe2x88x92STC) greater than 0 is true, correcting PCR to PCR+(Txe2x88x92xcex4) if |PCRxe2x88x92STC| greater than Txe2x88x92xcex4xe2x88x92xcex94 is true and sign(PCRxe2x88x92STC) greater than 0 is false, and making no correction if |PCRxe2x88x92STC| greater than Txe2x88x92xcex4xe2x88x92xcex94 is false, wherein the program clock reference received is represented by PCR, a system time clock at the time when the program clock reference is received is represented by STC, a period of a transport stream packet at an output terminal of an MPEG encoder is represented by T, a packet transmission time of the transport stream packet at an input terminal of the apparatus is represented by xcex4, and a tolerable range is represented by xcex94.
According to a second aspect of the present invention, there is provided an apparatus for reducing a jitter of a program clock reference in a transport stream of an MPEG signal transmitted over ATM system, which comprises: detection means for detecting a displacement of a reception timing of a transport stream packet containing the program clock reference with respect to a normal reception timing by a predetermined amount; and means of correcting a value of the program clock reference by the predetermined amount when the displacement is detected.
According to a third aspect of the present invention, there is provided an MPEG decoder which comprises the apparatus of the first or second aspect of the present invention.
According to a fourth aspect of the present invention, there is provided a method for reducing a jitter of a program clock reference in a transport stream of an MPEG signal transmitted over ATM system, which comprises: steps for correcting PCR to PCRxe2x88x92(Txe2x88x92xcex4) if |PCRxe2x88x92STC| greater than Txe2x88x92xcex4xe2x88x92xcex94 is true and sign(PCRxe2x88x92STC) greater than 0 is true, correcting PCR to PCR+(Txe2x88x9267 ) if |PCRxe2x88x92STC| greater than Txe2x88x92xcex4xe2x88x92xcex94 is true and sign(PCRxe2x88x92STC) greater than 0 is false, and making no correction if |PCRxe2x88x92STC| greater than Txe2x88x92xcex4xe2x88x92xcex94 is false, wherein the program clock reference received is represented by PCR, a system time clock at the time when the program clock reference is received is represented by STC, a period of a transport stream packet at an output terminal of an MPEG encoder is represented by T, a packet transmission time of the transport stream packet at an input terminal of the apparatus is represented by xcex4, and a tolerable range is represented by xcex94.
According to a fifth aspect of the present invention, there is provided a method for reducing a jitter of a program clock reference in a transport stream of an MPEG signal transmitted over ATM system, which comprises: a step for detecting a displacement of a reception timing of a transport stream packet containing the program clock reference with respect to a normal reception timing by a predetermined amount, and a step for correcting a value of the program clock reference by the predetermined amount when the displacement is detected.
A system time clock regenerator included in an MPEG decoder which receives a program clock reference outputted from the PCR jitter reducing apparatus of the present invention can reproduce a system time clock which is not affected by a timing error of the program clock reference and thus has no jitter because the timing error of the program clock reference is corrected by the value thereof in the PCR jitter reducing apparatus of the present invention.